Oil well cementing is a complex operation with multiple goals: mechanically secure the casing strings in the geologic formation, isolate a producing layer from adjacent layers, protect the strings against the corrosion due to the fluids contained in the layers crossed through. Under certain hydrocarbon reservoir development or geothermal conditions, it is essential to have cementing materials with low densities. In fact, when the formations drilled are fragile and unconsolidated, it is impossible to carry out operations with a cement slurry of conventional density, of about 1900 kg/m3, for fear of exceeding the drilled formation fracture pressure, which leads to considerable losses in the formation. This case is notably encountered when cementing the casing of offshore wells or of wells drilled in depleted mature fields.
In order to lower the density of cementing slurries, it is common practice to increase either the amount of water or the amount of gas. When the material is lightened by increasing the amount of water, this high water proportion generates a high porosity which translates into poor properties of the cement sheath in terms of permeability, mechanical strength, cracking and durability. When the solution selected consists in lightening the slurry using gas, there are two options. The first solution consists in adding hollow balls in the formulation of the cementing material. This technique is illustrated by documents U.S. Pat. No. 3,804,058, U.S. Pat. No. 4,252,193 and U.S. Pat. No. 5,019,976. The second solution consists in intentionally driving a large amount of gas into the cement slurry so as to cause it to foam. This technique is illustrated by documents U.S. Pat. No. 5,806,594 and U.S. Pat. No. 5,484,019. This cementing material is then referred to as foamed cement. The quality of the foamed cement represents the volume fraction of gas present in the slurry. Foamed cements with qualities that do generally not exceed 80% are used for cementing with foamed cement. For the success of cementing with foamed cement, it is particularly important to check that the foaming agent used allows to obtain foam qualities compatible with the desired density (this characteristic is referred to as “foamability” of the cement slurry).
Furthermore, the foam obtained has to be stable (this characteristic is referred to as foam “stability”). In fact, unstable cement foams will lead to the formation of a highly irregular cementing material where the pores of the porous structure are greatly connected, and which is therefore very permeable. The stability of the foamed cement is thus essential to obtain high-quality cementing meeting all the requirements relative to oil and gas well cementing.
To obtain foamability and stability of the foam of cementing materials (cement paste, mortars or concretes), a foaming agent (or a mixture of foaming agents) and foam stabilizing additives are generally used in combination. It is essential to correctly select the foaming agents and the foam stabilizing additives. The following criteria to be taken into account can be mentioned: efficiency in terms of foamability and stabilization of the foam, effect on the mechanical strengths and the permeability.
The foaming agents commonly used for foaming cement slurries are anionic surfactants or a mixture of surfactants. For example, it is well-known that a mixture of anionic surfactants and of non-ionic surfactants is efficient to obtain stable foams. The foam stabilizing additives can be either surfactants or polymers, long-chained alcohols, in liquid form or in solid particles.
One difficulty in foaming a cement material lies in the fact that the interstitial solution of these cement materials has a high pH value, often ranging between 12 and 13, and high ionic concentrations notably due to the calcium ions concentration. It is well-known that the surfactants used as foaming agents have a low efficiency with a basic pH value. It is also known that the presence of multivalent ions such as, for example, the calcium resulting from the dissolution of cement in water, has the effect of precipitating the anionic surfactants. In this case, the surfactants that have precipitated can no longer take part in the formation of the foam. One understands that foaming a cement slurry is not a trivial operation and that it is not simple to obtain stable good-quality foams.
The present invention aims to use hydrosoluble polymers comprising hydrophobic links for foaming cement slurries so as to obtain stables foams of good quality.